1. Field of the Invention
The present invention relates to a write-once optical recording medium which employs an organic dye for a recording layer. Particularly, it relates to a novel compound corresponding to blue laser, a write-once optical recording medium and an optical recording method.
2. Discussion of Background
At present, various optical recording media such as CD-R/RW, DVD-R/RW and MO have been widely recognized and used as an external memory for an information processor such as a computer, since they can record a large amount of information, and random access is easy with them. Among these optical recording media, organic dye type optical recording media typified by CD-R and DVD-R are considered to be superior to others from such a viewpoint that they can easily be produced at a low cost.
Further, along with increase in the amount of information to be recorded, it has been desired to increase the recording density of media. In recent years, an optical recording medium on which high density recording/readout can be carried out by using laser with a short oscillation wavelength, such as blue laser which has significantly been developed, has been proposed.
In general, with respect to optical recording media commercially available as CD-R or DVD-R, for example, CD-R is designed to be applicable to recording/readout by a laser beam having a wavelength at a level of 780 nm, and DVD-R is designed to be applicable to recording/readout by a laser beam having a wavelength at a level of from 600 to 700 nm. Such a recording medium applicable to optical recording/readout by using a laser beam having a relatively long wavelength has such problems that when recording/readout is carried out by using laser with a shorter wavelength, the refractivity tends to be low, and recording/readout cannot be carried out. It is an object of the present invention to provide an organic dye type optical recording medium on which recording/readout can be carried out by blue laser with a shorter wavelength.
The present inventors have conducted various studies on organic dyes having a high sensitivity to blue laser and as a result, they have found that compounds of the following formulae [I] and [II] can be used for a recording layer of an optical recording medium corresponding to blue laser, and the present invention has been accomplished on the basis of this discovery.
Namely, the present invention provides an optical recording medium comprising a substrate and a recording layer on which recording and readout of information can be carried out by laser, formed on the substrate, wherein the recording layer contains a compound of the following formula [I] or [II], and an optical recording method, which comprises recording information on the optical recording medium by using a laser beam having a wavelength of from 350 to 530 nm: 
wherein each of R1 to R4 is a hydrogen atom or an optional substituent, provided that R1 and R2, and R3 and R4, may be bonded to form a hydrocarbon ring or a heterocyclic ring structure, and the hydrocarbon ring and the heterocyclic ring may have a substituent;
X1 is an electro-withdrawing group, and X2 is a hydrogen atom or xe2x80x94Qxe2x80x94Y (wherein Q is a direct bonding, a C1-2 alkylene group, an arylene group or a heteroarylene group, and Y is an electro-withdrawing group, provided that the alkylene group, the arylene group and the heteroarylene group may have an optional substituent in addition to Y); and
Z is xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SO2xe2x80x94 or xe2x80x94NR5xe2x80x94 [wherein R5 is a hydrogen atom, a hydrocarbon group which may be substituted, a heterocyclic group which may be substituted, a cyano group, a hydroxyl group, an amino group represented by xe2x80x94NR6R7 (wherein each of R6 and R7 which are independent of each other, is a hydrogen atom, a hydrocarbon group which may be substituted, a heterocyclic group which may be substituted or xe2x80x94COR8  less than wherein R8 is a hydrocarbon group which may be substituted or a heterocyclic group which may be substituted greater than ) or xe2x80x94COR9 (wherein R9 is a hydrocarbon group which may be substituted or a heterocyclic group which may be substituted), provided that when R5 is a hydrocarbon group, it may be bonded with R2 or R4 to form a cyclic structure]; 
wherein each of R1 to R4 is a hydrogen atom or an optional substituent, provided that R1 and R2, and R3 and R4, may be bonded to form a hydrocarbon ring or a heterocyclic ring structure, and the hydrocarbon ring and the heterocyclic ring may have a substituent;
the ring A is a carbocyclic ketone ring or a heterocyclic ketone ring which may have a substituent, formed together with Cxe2x95x90O; and
Z is xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SO2xe2x80x94 or xe2x80x94NR5xe2x80x94 [wherein R5 is a hydrogen atom, a hydrocarbon group which may be substituted, a heterocyclic group which may be substituted, a cyano group, a hydroxyl group, an amino group represented by xe2x80x94NR6R7 (wherein each of R6 and R7 which are independent of each other, is a hydrogen atom, a hydrocarbon group which may be substituted, a heterocyclic group which may be substituted or xe2x80x94COR8  less than wherein R8 is a hydrocarbon group which may be substituted or a heterocyclic group which may be substituted greater than ) or xe2x80x94COR9 (wherein R9 is a hydrocarbon group which may be substituted or a heterocyclic group which may be substituted), provided that when R5 is a hydrocarbon group, it may be bonded with R2 or R4 to form a cyclic structure].
The present invention further provides a novel compound of the following formula [Ixe2x80x2]: 
wherein each of R1 to R4 is a hydrogen atom or an optional substituent, provided that R1 and R2, and R3 and R4, may independently be bonded to form a hydrocarbon ring or a heterocyclic ring structure, and the hydrocarbon ring and the heterocyclic ring may have a substituent; each of X1 and Y is an electro-withdrawing group, provided that one of X1 and Y is a cyano group, and the other is xe2x80x94COOR11 (wherein R11 is a hydrogen atom, a hydrocarbon group which may be substituted or a heterocyclic group which may be substituted) or xe2x80x94SO2R17 (wherein R17 is a hydrocarbon group which may be substituted or a heterocyclic group which may be substituted); and
Z is xe2x80x94NR5xe2x80x94 (wherein in a case where one of X1 and Y is xe2x80x94COOR11, R5 is a C3-18 branched alkyl group which may be substituted, a C3-18 cyclic alkyl group which may be substituted or a 5- or 6-membered saturated heterocyclic group which may be substituted, and in a case where one of X1 and Y is xe2x80x94SO2R17, R5 is a C1-18 linear or branched alkyl group which may be substituted, a C3-18 cyclic alkyl group which may be substituted or a 5- or 6-membered saturated heterocyclic group which may be substituted).